Dedicated xml,json,yang and xsl parser utility programs added

Sanity check of stdarg (...) added
Cleanup of error messages.

apps/backend/backend_plugin.c

@@ -169,7 +169,7 @@ clixon_plugin_statedata(clicon_handle        h,
 	}
     }
     /* Code complex to filter out anything that is outside of xpath */
-    if (xpath_vec(*xtop, xpath?xpath:"/", &xvec, &xlen) < 0)
+    if (xpath_vec(*xtop, "%s", &xvec, &xlen, xpath?xpath:"/") < 0)
 	goto done;
 
     /* If vectors are specified then mark the nodes found and